Mike, ZL1BNB wrote:
"I`ve got a simple UHF return loss bridge and a counter.

What else do I need?"

To measure the resonant frequency of the 450 MHz dipole, you need an
appropriate variable frequency signal source.

Return loss is the difference between incident and reflected powers at
the antenna. At resonance, antenna reactances cancel removing a large
impediment to power acceptance by your antenna.

Energy absorbed by the antenna is not reflected back toward the signal
source so you have the greatest difference between incident and
reflected powers near resonance (greatest return loss).

Best regards, Richard Harrison, KB5WZI